Antigenically naive CD4 expressing a-T cell receptors are critical for generating immune responses to neoantigens, and are produced de novo within the thymus as mature CD4+CD8- thymocytes. These enter the periphery to become recent thymic emigrants (RTEs) of the naive CD4 T-cell compartment. Although monitoring of CD4 RTEs is of great clinical interest, particularly in CD4 T-cell lymphopenia, direct evaluation of human CD4 RTEs has been limited by a lack of specific surface markers. This project will utilize a novel and recently identified surface marker for human CD4 RTEs, protein tyrosine kinase 7 (PTK7), to define CD4 RTE frequency, phenotype, and function. Preliminary results validate PTK7 as a CD4 RTE marker, and show that PTK7+ CD4 RTEs have a reduced capacity for effector function compared to PTK7- naive CD4 T cells. Aim 1 will use gene expression profiling by deep mRNA sequencing of unstimulated and stimulated PTK7+ CD4 RTEs and other CD4 T-lineage cells to: 1) define the extent of residual thymocyte gene expression in PTK7+ CD4 RTEs during ontogeny, and 2) identify gene products involved in reduced CD4 RTE immune function, particularly for Th1 generation and/or that subdivide PTK7+ CD4 RTEs into more versus less recent thymic emigrants. Aim 2 will use in vivo labeling to test the hypothesis that PTK7+ CD4 RTEs are the direct precursors of PTK7- naive CD4 T cells, and will determine the role of PTK7+ CD4 RTEs in maintaining naive CD4 T-cell numbers and 12-TCR repertoire diversity in HIV-1 infection. Aim 3 will use a similar approach to test the importance of PTK7+ CD4 RTEs in immune reconstitution of naive CD4 T cells after allogeneic hematopoietic stem cell transplantation. Aim 4 will define the kinetics of loss of PTK7+ CD4 RTEs in children and adults following complete thymectomy and the impact of this loss on naive CD4 T-cell numbers and 12- TCR repertoire diversity. Together, these studies will substantially enhance our understanding of the role of thymic RTE production in maintaining the peripheral naive CD4 T-cell compartment in health and disease.